Seagate announced its Pulsar SSD line on December 7 or 8 (depending on which version of the press release Google finds for you), allowing a show to drop that people had expected for more than a year. Pulsar drives use the familiar 2.5-inch HDD form factor and a SATA interface, making it easy to drop the drives into existing computer and server systems. Seagate’s Pulsar SSDs employ SLC (single-level cell) NAND Flash devices, which cost more per bit than MLC (multi-level cell) and TLC (three-level cell) NAND Flash devices. In exchange for the higher cost, you get more reliable memory, as was discussed in this blog a while back. (Check out “More than Moore: SLC, MLC, and TLC NAND Flash.”)

Seagate Pulsar SSD

The use of SLC NAND Flash underscores Seagate’s focus on enterprise-class storage for the SSD. There are at least two good reasons for Seagate’s enterprise focus. First, enterprise customers are more able to translate an SSD’s speed advantage over HDDs into dollars (as previously discussed in the blog entry “SSD TCO (Total Cost of Ownership”). Second, SSDs are a premium product with a premium price. Enterprise customers more easily accept the higher cost/Gbyte price tag attached to SSDs. Seagate’s Pulsar SSDs are available in storage capacities to 200 Gbytes and the SSDs achieve “a peak performance of up to 30,000 read IOPS and 25,000 write IOPS, 240MB/s sequential read and 200 MB/s sequential write” according to Seagate’s press release. The Pulsar drives have a 5-year limited warranty.

Micron Technology rolled out its RealSSD C300 less than a week before Seagate’s SSD announcement. The first glaringly obvious difference in Micron’s C300 SSD is that it sports a 6-Gbyte/sec SATA 6.0 interface. However, the faster interface alone will not boost performance (discussed earlier in this blog here) if the drive internals aren’t designed to sustain high transfer rates supported by SATA 6.0. To that end, Micron’s RealSSD C300 press release discloses the fact that the new Micron SSD “leverages a finely tuned architecture and high-speed ONFI 2.1 NAND Flash  to provide a whole new level of performance.” (ONFi, the Open NAND Flash interface, is discussed in this previous blog entry.) The result: a read throughput speed of up to 355MB/s and a write throughput speed of up to 215MB/s.

Compare those numbers to Seagate’s Pulsar and you’ll see that the Micron drive’s read throughput is nearly 50% faster but the write throughput is only 7.5% faster. Write throughput is one of the Achilles’ heels of SSDs. NAND Flash devices had an erase/write cycle that simply takes time.

Micron RealSSD C300

Micron’s C300 SSDs will be offered in 1.8-inch and 2.5-inch form factors, with both form factors supporting 128- and 256-Gbyte capacities. Micron is currently sampling the C300 SSD in limited quantities and expects to enter production in the first quarter of calendar 2010.

Both companies are making smart moves into the SSD market. Seagate, like Western Digital and its acquisition of SSD vendor SiliconSystems in March of this year, recognizes that it’s not in the HDD business—it’s in the storage business and SSD storage is hot right now. Micron, like Intel, sees SSDs as a value-added way to package and market it’s NAND Flash devices. Both companies have made very smart moves into the SSD market.

Find those last two sentences confusing or contradictory? Here’s the explanation.

SSDs have no read/write heads or positioning arms. Instead, they consist of several NAND Flash chips and a controller chip. There’s an array of memory blocks on each NAND Flash chip. The size of the Flash memory block is the smallest amount of memory a NAND Flash chip can write in one operation because NAND Flash memory blocks are atomic with respect to erasure. You can’t write just one byte or word because you must erase the entire block before writing to the block. That means an SSD can only write an entire NAND block at a time.

Here’s a graphic from Jim Handy’s SSD keynote at the Bell Micro SSD seminar held in early December that helps to explain the situation:

Each SSD consists of a stack of visible NAND memory blocks that the SSD controller uses to store written data. There’s also a shorter stack of spare NAND memory blocks that hold data in temporary storage. These spare blocks are also used to replace a visible block when it wears out from repeated write/erase cycles. All NAND blocks are equally accessible, so there’s no time penalty for writing NAND blocks out of sequence as there is when writing on non-adjacent or non-contiguous tracks with HDD storage.

However, most virtual operating systems don’t write in blocks, they write in 4-Kbyte pages that are much smaller than NAND Flash blocks. For example, Numonyx’ 1-to-16-Gbit NAND Flash devices have 128-Kbyte blocks. As a result, modifying one 4-Kbyte page in a NAND Flash block requires a relatively complex sequence:

1. Read the data for the entire block from NAND Flash into a RAM buffer
2. Modify the appropriate page in the block image now stored in RAM
3. Write the block back to an erased NAND Flash block
4. Fix pointers to the new memory block
5. Erase the old memory block as a background task

Consequently, SSD performance varies over time and the performance varies depending on how many erased and spare memory blocks are available across all of the NAND Flash chips in the SSD. SSD performance also depends on the ratio of reads versus writes—because reads occur ten times faster than writes for SSDS—and they vary over time as the NAND Flash chips fill up.

The following figure from Handy’s keynote shows a 3D data surface plot representing the IOPS performance of one SSD. (The figure is from a presentation at the August 2009 Flash Memory Summit made by Esther Spanjer, Director of SSD Marketing at Smart Modular Technologies.)

The X axis of the surface shows the ratio of reads to writes and varies from 100% writes on the left to 100% reads on the right. The Y axis shows SSD performance in IOPS. The Z axis plots “block” size, from the SSD-level perspective (which is page size from the NAND Flash chip’s perspective, yes that’s confusing).

The first thing to note from this surface plot is that performance is a lot better on the right-hand side, which is dominated by reads. You’d expect that because SSD read performance is 10x better than SSD write performance. It’s the nature of NAND Flash memory. Note how fast the performance falls off as the percentage of write transactions increases. Then note that there’s a sort of saddle effect along the Z axis. The saddle peaks at 4-Kbyte blocks. Most SSD designs are optimized for 4-Kbyte blocks because most virtual operating systems employ 4-Kbyte blocks (and have for decades, in spite of the radical, orders-of-magnitude increase in memory use by both operating systems and application software).

So, clearly, when an SSD vendor gives an IOPS rating for an SSD, you need to take that one number with a grain of salt. SSD performance varies significantly depending on the read/write mix and on block size. Consequently, SSD performance can’t be captured in one or two numbers.

Next, Handy presented this graphic from SandForce (which makes SSD controller chips):

This graph shows an initial conditioning period during which the test preconditions (fills up) the SSD using sequential 128-Kbyte writes. The initial transfer performance (about 80 Mbytes/sec for the particular drive being tested) drops slightly as the drive fills and the internal SSD controller starts shuffling full NAND Flash blocks off to spare memory. The falloff isn’t big because the sequential writes place a predictable load on the SSD controller. However, when the test switches to random 4-Kbyte writes about 4000 seconds into the test, performance drops significantly because the SSD controller suddenly needs to make small changes to memory stored in the NAND Flash blocks but the drive’s full and there are no empty blocks. Blocks must be erased to make room for the new data and block erasure takes time. Consequently, there’s a big performance falloff as the controller starts to shuffle data around inside of the drive to make room for new data.

Perhaps more interesting is what happens when the test switches back to large sequential writes about 11,000 seconds into the test. Initially, the sequential writes cause the drive performance to vary wildly because the preceding random writes have scattered the spare blocks and left them distributed throughout the SSD’s internal NAND Flash memory space. Eventually, the SSD’s internal controller gets things sorted out and the performance for large sequential writes returns to the initial steady-state level.

(Note: This graph is not supposed to typify the performance of all SSDs. The graph shows the results of a test on one particular SSD.)

So what’s to be learned from all of this data? SSD performance measurement isn’t simple. Creating controllers and firmware that deliver optimum SSD performance isn’t simple either. As drive and chip vendors learn more about the use of NAND Flash for storage, they develop better algorithms for extracting more performance from the NAND Flash chips.

NAND Flash chips are complicated, whether used in SSDs or for server memory backup as with AgigA Tech’s AGIGARAM modules. It takes experience to get the most performance from these memory devices.

My thanks to Jim Handy for all of the great information in his Bell Micro keynote, and for generously letting me use the information in this series of blog entries.

First, take a look at this graphic depicting a “typical” server storage array consisting of 100 enterprise-class HDDs.

Each short-stroked, enterprise-class HDD has a capacity of 300 Gbytes, for a total array capacity of 30 Tbytes. This enterprise-class HDD array delivers 30K IOPS, costs $55,000, and consumes 1.75 kilowatts of electricity (not to mention an equivalent amount of electricity required for cooling). That’s the baseline.

Now look at this graphic, which compares the previously discussed array of enterprise-class HDDs with a hybrid array consisting of one SSD and 30 high-capacity, 1-Tbyte HDDs.

The high-capacity drives are not short-stroked, so they can provide a total storage capacity of 30T bytes with only 30 drives instead of 100 enterprise-class HDDs. However, the one SSD inserted into the drive array provides the same IOPS read performance as the 100 enterprise-class HDDs, so the use of the slower, less expensive, high-capacity HDDs in the second array is not a detriment to the second array’s IOPS performance, as long as the server software is written to make use of the hybrid array’s abilities.

The SSD-enhanced drive array costs $6040 or about 90% less than the array of 100 enterprise-class HDDs. The SSD-enhanced array consumes 0.392 kilowatts, which is nearly 80% less than the enterprise-class array of 100 short-stroked HDDs. Consequently, the second drive array generates substantially less waste heat (that must be cooled) than the full array of enterprise-class HDDs.

As a result, the SSD-enhanced drive array saves the enterprise customer a substantial amount of money when viewed from a systemic perspective. Relative to the enterprise-class HDD array, the SSD-enhanced hybrid drive array costs less to purchase; costs less to provision because fewer drives require less rack space and fewer racks; consume less electricity for operation; need less electricity for cooling because fewer, slower drives generate less heat; and reduce maintenance costs because the high-capacity drives run cooler (increasing MTBF), because there are fewer drives to maintain, and because high-capacity drives are much less expensive than enterprise-class drives. Overall, the TCO calculations favor the SSD-enhanced, hybrid drive array.

Handy took Sun’s numbers a step further by calculating the crossover point where TCO considerations favor an SSD-enhanced drive array over an array of enterprise-class HDDs when the IOPS performance is the main consideration rather than capacity. Here are Handy’s graphs:

The left graph shows price curves for an array of enterprise-class HDDs versus an array of SSDs. The array of SSDs initially costs more than the enterprise-class HDDs, so the crossover point is 1200 IOPS due to the high initial SSD cost. As the IOPS requirement rises, you need to add enterprise-class HDDs to the array to meet the higher IOPS requirements but one SSD gets you a lot of IOPS so there’s no need to add one until the IOPS requirement exceeds around 3000 IOPS. For the enterprise-class hybrid array, which mates one SSD with several high-capacity HDDs, the purchase cost of the SSD-enhanced array is much lower for a given capacity so the crossover point is also lower—just 400 IOPS.

TCO computations such as these are required for storage and for memory subsystems. It’s easy to be myopic and compare component cost to component cost, but system architects are creating systems and should always try to view component costs through a TCO lens.

Why are the solid state disk drives still so expensive?

They are on the market for years and still so expensive. SSD of a reasonable capacity (256GB) costs as much as $800 or more. Aren’t they going to drop the prices?

Although the question appears to have been posed by someone not closely familiar with the ins and outs of hard-disk drive (HDD) and solid-state disk (SSD) technologies, markets, and pricing, it’s a frequent question posed by many in the industry. We’ve become so accustomed to large, regular drops in price/capacity for both mechanical storage (“rotating rust”) and semiconductor memory that we’ve collectively developed a sense of entitlement. If we can’t buy it today, we think, surely the price will drop and we’ll be able to afford it soon.

However, when we compare the price/capacity of SSDs against HDDs, we’re comparing one moving target against another. Moore’s Law governs the price of SSDs because the largest cost component in an SSD is NAND Flash memory (see below). Moore’s Law has been a monster force in the semiconductor industry, pushing prices ever lower for more than four decades. However, the HDD vendors are constantly working with their own price-reduction curve, which has proven to be just as robust as Moore’s Law. By pulling a veritable menagerie of rabbits out of various technological hats, HDD vendors have dropped per-bit pricing for HDDs about as fast as semiconductor vendors have cut the price/bit of NAND Flash memory.

Take a look at this graph from Handy’s keynote:

From the gross slopes of the two curves, you can see that HDD cost/capacity has remained about 20x lower than NAND Flash memory cost/capacity throughout this decade. Note that in 2006, there was a serious downturn in the slope of the curve for NAND Flash. Extrapolating that new slope led some to predict that NAND Flash cost/Gbyte would cross over that of HDDs by 2008 or 2009. That just didn’t happen. The increased rate of price decline was economically unsupportable and caused huge turmoil among NAND Flash vendors. (For extensive analysis of this situation, see this blog entry on Denali Software’s Web site.)

Now please understand, the expectation that NAND Flash cost/Gbyte would zoom past the HDD cost/Gbyte curve wasn’t just wishful thinking. NAND Flash per-bit costs did overtake and then zoom past that of DRAM, which was once the semiconductor industry’s king of cost/bit. That event happened in 2004 as shown in this slide from Handy’s keynote.

So the expectation that NAND Flash cost/bit would zoom past HDD cost/bit wasn’t at all far-fetched. It just didn’t happen. HDD vendors happily continued to cut the cost/bit of rotating storage, to the very great benefit of consumers and enterprise users everywhere.

Handy’s simple silicon anatomy of an SSD shows why the SSD’s cost/bit is closely tied to the cost of NAND Flash.

From a silicon perspective, Handy’s illustration shows 34 key semiconductor devices in his example 64-Gbyte SSD. Two of the devices are a controller chip and a DRAM buffer. Total cost for those two devices: $6. The other 32 devices are NAND Flash chips. Total cost for those devices: $64 for 64 Gbytes of storage (not counting spare capacity). The cost of the NAND Flash devices is more than 90% of the silicon cost of an SSD. The SSD’s price is largely set by the cost of its internal NAND Flash.

That’s why SSDs aren’t likely to replace HDDs for bulk storage in the foreseeable future. As long as the HDD industry has a road map leading to higher capacity and lower cost/bit storage, and it does, then the HDD will keep the throne as the storage capacity king.

SSDs can beat HDDs in raw performance by one or two orders of magnitude, as measured in IOPS. There’s nothing on the HDD road map that can change that situation. For applications that can measure the value of storage speed, and there are many such applications for enterprise-class storage, SSDs provide sufficient value to justify their higher price/bit. For most consumers, people who are selecting laptops for example, the choice between a 160-Gbyte HDD or a 32-Gbyte SSD for the same price is obvious. The consumer will choose more capacity (to store more music, more pictures, more video, and more movies) every time.

Now take a look at Handy’s curves for DRAM and NAND Flash cost/bit once again:

Note that the cost/bit of NAND Flash is now roughly 10% that of DRAM. That means that as a DRAM backup medium, NAND Flash doesn’t add that much to the cost of the DRAM it’s backing up. Unlike the comparison of NAND Flash and HDD capacity, which tilts far in favor of the HDD, NAND Flash densities are much better than DRAM bit densities and that gap is growing thanks to multi-level cell (MLC) storage. These economics are behind the idea for AgigA Tech’s AGIGARAM modules. For a small cost adder, volatile DRAM can be made bulletproof when paired with NAND Flash memory. For more detail regarding this idea, see the earlier 3-part series in this blog (here, here, and here).

One of the interesting facets Handy discussed was the current practice of short-stroking enterprise-class hard disk drives (HDDs)—“abusing” them, as Handy explained. The idea’s pretty simple. An HDD’s average access time is determined by the average amount of time it takes to swing the arm carrying the read/write heads into position plus the average rotational latency. The fastest enterprise-class HDDs now spin at 15,000 RPM so there’s not much room for trimming there—not without having the disk platters fly apart under the centripetal force. However, there’s something that can be done about the average seek time. Simply use fewer of the available tracks on the disk. Doing so, you get faster average seek times because the arm never needs to travel very far.

You pay for that decreased seek time with lost capacity. You simply don’t use most of the tracks and therefore you discard most of an HDD’s storage capacity.

Handy gave the following real-world example of such HDD abuse. He described IBM’s DS8300 Turbo. It has best-in-class TPC-C specs: 123K IOPS, 16-msec latency. It gangs 512 HDDs—consisting of 73- and 146-Gbyte enterprise-class drives—into mirrored RAID arrays. The result is a storage subsystem with 53 Tbytes of actual capacity, but short-stroking the drives reduces the usable capacity to 9 Tbytes. IBM threw away 83% of the raw capacity to get those best-in-class TPC-C performance specs.

This is yet another example of why SSD manufacturers are crowding into the Flash Zone. If IBM can afford to throw away 83% of the available capacity in a huge multi-multi-Tbyte bank of enterprise-class HDDs, then high-performance SSDs that can muster one or two orders of magnitude performance improvement relative to the “rotating rust” HDDs must be worth a lot of money to data-center architects.

And apparently, they are.

For this blog entry, we’re returning to the Flash Zone, a concept described by Denali Software’s CTO Mark Gogolewski in his keynote speech—The World is Flash: A Disruption of the Memory & Storage Hierarchy—at Memcon 2009. The Flash Zone is the name put to the performance gap between DRAM and disk storage. There’s not only a gap in performance within the Flash Zone, there’s a transition from volatile memory (DRAM) to non-volatile storage (hard disk). With steep cost/bit price declines and per-device capacity growth, NAND Flash devices now easily fit into this gap and produce a new and viable layer in the overall computer memory hierarchy.

What’s new is that Jim Handy’s keynote at the Bell Micro SSD seminar put some welcome numbers on the Flash Zone that further clarify Flash’s place in the hierarchy. Here’s an image of that particular slide.

This image plots the performance and cost of the different memory hierarchy layers from first-, second-, and third-level processor cache through DRAM, disk, and tape. Because Handy’s used a log-log scale to plot everything, the graph looks nice and linear even though the reality is quite a bit messier. For a conceptual graph however, this’ll do nicely.

Note that there’s a gap in the hierarchy. That’s the Flash Zone. Here’s the same plot augmented a bit. The big red circle identifies the Flash Zone.

Also note that Handy has labeled the gap and says it’s “growing.” The gap’s growing because DRAM is getting faster, bigger, and cheaper, moving its ellipse up and to the left while HDDs are getting bigger, although not much faster, moving the HDD ellipse horizontally to the left. The result is a growing performance and bandwidth gap between DRAM and HDDs.

Flash fits into this gap very, very nicely said Handy (and as discussed in this blog previously). Later in his keynote, he displayed this image to underscore the point.

There are currently at least three ways to fill the Flash Zone in a memory hierarchy using NAND Flash memory. The first way, the way that gets the most attention these days, is with solid-state drives (SSDs). Because they employ the same interfaces and share the same form factor with HDDs, SSDs are an easy, drop-in Flash Zone filler. They boost performance just by dropping them into place as HDD replacements, although that may not be the best way to introduce SSDs into the hierarchy. (More about that in a later blog.)

The second way to drop NAND Flash memory into the Flash Zone is through direct- or I/O-attached drives. This is the approach advocated by Fusion-io, as discussed in that earlier AgigA Tech blog entry on the Flash Zone. Direct-attached SSDs eliminate the HDD interface and protocols, which were designed with built-in assumptions about the performance characteristics and limitations of HDDs (“rotating rust” quipped Scott Stetzer, VP of Marketing at SSD vendor STEC). Free of those limiting assumptions and limits, direct-attached SSDs deliver more performance than do SSDs employing HDD interfaces.

Handy showed the ways to introduce these two types of SSDs with the following slide:

In enterprise-class server systems, SSDs with HDD interfaces typically plug into SAN racks and tie to servers over a network while direct-attached SSDs plug directly into the server over a high-speed interface (typically PCIe). Note that smaller servers with HDD interfaces often talk to SSDs directly.

Because he was speaking at an SSD seminar, Handy did not discuss the third way of introducing NAND Flash into the Flash Zone—the approach employed by AgigA Tech’s AGIGARAM. That approach mates the NAND Flash directly to the server’s DRAM, creating a high-bandwidth connection between the two memory hierarchies. In this application, however, the NAND Flash is used for DRAM backup and power-failure bulletproofing—not necessarily for storage (although there are other possibilities to be discussed in this respect).

So far, we’ve only been able to discuss two of Handy’s 47 keynote slides. The talk contained a ton of good information for server designers and enterprise system architects. More later.

Note: Handy’s keynote was based on his company’s new report: Solid State Drives in the Enterprise – 2010.

SSDs are one of three ways to fill the memory/storage gap called the “Flash zone” as discussed in the previous AgigA Tech blog entry, which described Flynn’s initial panel presentation. Although not a major consumer of NAND Flash memory devices, yet, SSD use is growing quickly because of the speed advantages they deliver over what can be achieved with rotating mechanical storage (hard disk drives). Flynn’s talk described the ideal conditions under which I/O-attached storage (including products offered by Fusion-io) can deliver stellar storage performance as measured in IOPS. Flynn’s presentation prompted the first panel question from moderator Coughlin: “Are hard disk drives dead?”

Flynn answered first. Unsurprisingly, he said “No.” Tape hasn’t died either, said Flynn, and neither has DRAM. None of these technologies is in danger of disappearing overnight. HDDs (hard disk drives) currently enjoy a huge cost/capacity lead over any competing storage technology (excluding tape) and HDDs will only disappear when they lose that lead.

Speiser also weighed in. Tape’s huge cost/capacity lead over HDD storage is the only factor that keeps tapes alive for their ultimate use: “offline storage inside of (hollowed-out) mountains.” Tapes will outlast HDDs added Speiser. “They’re the cockroaches of the storage industry.” Chenery, who left HDD vendor Fujitsu in 2006 to start SSD supplier Pliant, also spoke favorably about HDDs. “No one wants a mechanical drive in their computer,” said Chenery, because of the power consumption and susceptibility to physical shock. However, “they provide so much value for capacity” he explained. “In 30 years, who knows?”

Watkins disagreed. “No one cares what’s in their PCs. Consumers think about the applications they want to run. Then they find the best hardware to fit their needs.” Watkins is more concerned by the applications that consumers will be using in five years. His conclusion: all mobility products will evolve into Flash-only use because Flash memory provides superior form factors for small, mobile end products. Meanwhile, cloud storage may obsolete large HDDs in laptops because it’s too dangerous to carry around all that valuable data in a form where it can be lost, stolen, damaged, or destroyed. Yahoo’s Pullara smiled at Watkins’ comment about cloud storage and quipped “How about unlimited storage (in Yahoo’s cloud)? Can you beat that, Google?”

“So if HDDs aren’t going away any time soon,” asked Coughlin, “why did you (Chenery) start Pliant?”

“Because no one would listen to me” replied Chenery, who feels that SSDs are clearly going to redefine they way computer systems are architected.

Speiser jumped on the bandwagon. “We’re looking to invest in companies that have fundamentally rethought applications to back out assumptions based on spinning media.”

Pullara concurred. “Look at anti-spam in 2003” he said. The need to maintain extensive lists of spam sources has soared since then. Maintaining those lists on slow HDDs would make it impossible to reject spam in real time, given the rising volume of spam emails.

Watkins returned the discussion to mobile applications. “The sweet spot for Flash is in the hand,” he said. SSDs must reach 100-Gbyte capacities for netbooks while enterprise applications require terabytes of data storage and corresponding changes in server architecture.

The question of data reliability and trust then arose. Flash memory has well-documented, well-understood wearout and failure mechanisms. In fact, Flash vendors have been far more open and informative about these technology issues than have HDD vendors. As a result, people better understand Flash failure modes and are more aware of them. Chenery grinned and asked “Why would you trust your data to a flying head on a disk?” referring to the incredibly small gap between the read/write head and the spinning media. Head crashes are a well-known HDD failure mechanism. “Flash memory has its idiosyncrasies, but technology overcomes a lot of these” said Chenery. “You manage these idiosyncrasies with appropriate controllers, software, and use models.” In the end said Chenery, system-level designers shouldn’t trust any of the HDD or SSD vendors. They should test and verify reliability claims.

In addition, said Chenery, SSDs don’t “fall off the cliff” (fail catastrophically like HDDs). They provide deterministic, predictable performance that allows for soft failures, usually seen as a gradual capacity decrease as control firmware walls off bad blocks in the Flash memory and moves data to good blocks. Most SSDs will decline in performance over time, claimed Chenery. They must be designed specifically to not decline in performance at the subsystem level. “Getting Flash to deliver deterministic performance in a random environment is hard. It requires enormous computing power.”

There’s a similar but even larger performance gap between the access time of DRAM and HDDs. Although vendors have improved HDD capacity by 60% per year—each and every year—and HDD’s price per storage bit directly tracks that trend as well, there’s been very little improvement in HDD data transfer rate and interface speed and there’s been no dramatic change in HDD access time, which is largely determined by mechanical factors. Consequently, there’s been only a relatively slow improvement in HDD IOPS (I/O operations per second), which leads to a massive five-orders-of-magnitude (10^5) performance gap between DRAM access times and HDD access times and that performance gap is growing.

At the same time, DRAM’s volatility plays a role in a system’s sensitivity to power glitches and losses. When data is critical, and most data is critical these days, non-volatile memory just isn’t sufficient. Some means of retaining data through a power loss is usually required. In the past, HDDs have sufficed for non-volatile storage but they’re simply too slow these days.

The Flash Zone

Flash memory is a good candidate for filling this memory gap because it provides nonvolatile storage and it has become the cost-per-bit leader in semiconductor memory. Consequently, Mark Gogolewski, Denali Software’s CTO, calls this performance gap in the memory hierarchy the “Flash Zone” (see figure below and the Reference), the performance zone between DRAM and HDD access times.

The Flash Zone in memory hierarchy

The reasons for Flash memory’s candidacy to fill this gap include:

• In 2004, the per-bit cost of NAND Flash dropped below the previous category leader, DRAM.
• More NAND Flash bits shipped in 2005 than bits of any other type of semiconductor memory.
• More NAND Flash bits shipped in 2007 alone than all of the DRAM bits shipped in the last 25 years of commercial DRAM production.

There’s been a huge decrease in the per-bit cost of NAND Flash and a big capacity increase on a NAND Flash die. Consequently, NAND Flash memory fits nicely in the gap between DRAM and HDD. It offers faster access speeds than HDDs by at least two orders of magnitude while replicating an HDD’s non-volatile storage abilities. In addition, NAND Flash memory can draw considerably less power than HDDs when managed correctly. The opportunity for innovation in memory hierarchy is therefore huge.

Three Ways to Use NAND Flash: SSDs, Flash Cache, DRAM Backup

There are three ways to fill the Flash zone. The first approach is to use NAND Flash memory to create an HDD emulator using the same disk interface and possibly even the same form factor. Such drives are called solid-state drives (SSDs) and they have been gaining traction in the industry. Because they do not employ rotating memory, SSDs can deliver far faster access times than HDDs. However, there are costs associated with this approach. SSDs cost substantially more per stored bit than HDDs while retaining the overhead associated with HDD interfaces and protocols. The memory bus protocols and interfaces used to connect DRAMs to processors are much, much faster.

At this time, most analysts agree that NAND Flash memory will not overtake HDDs in cost per bit. Jim Handy of Objective Design presented the chart shown below at MemCon 2008 showing that the 25x cost-per-bit advantage for HDDs relative to NAND Flash memory cost per bit would continue for the foreseeable future.

NAND Flash and HDD cost-per-bit forecasts
(Jim Handy, Objective Design)

Denali’s memory market analyst Lane Mason recently commented that the pace of cost-per-bit reductions for NAND Flash memory will actually slow compared to price drops in recent years. So it doesn’t appear that NAND Flash will supplant HDD storage in the near- or medium-term future.

The second way to use NAND Flash memory in the Flash Zone is called a Flash cache. A Flash cache speeds access to an HDD by buffering the data stream between a processor and the HDD. Data is drawn from and written to HDDs as needed and the same data is simultaneously cached in NAND Flash. The next time this data is needed, it’s drawn directly from the Flash cache instead of the slower HDD. Flash caches do not require as much NAND Flash memory as SSDs, and therefore cost less, but they can deliver performance improvements when paired with HDDs.

The third way to use NAND Flash memory is to implement a backup strategy that allows the DRAM to operate normally when system power is available and to quickly save that data in non-volatile NAND Flash when system power fails. In this approach, which is used in AgigA Tech’s AGIGARAM Non Volatile System (NVS) modules, a backup power source provides the energy needed to safely tuck data away in non-volatile storage (NAND Flash), which then retains the data for a decade or more if needed.

This third approach to filling the Flash Zone offers several benefits including:

1. Fast backup when power fails
2. No energy required to save the data during power failure
3. Automated backup and restoration of data with no host-based software assist required

Which of these three approaches to use depends on the application (as always). If you’d like help deciding, please feel free to contact AgigA Tech.

Reference

The World is Flash: A Disruption of the Memory & Storage Hierarchy, Keynote Speech, Denali Memcon 09, Mark Gogolewski, CTO, Denali Software, Inc., www.denali.com